A conventional permanent magnet motor is simple in structure, stable in operation, small in size, low in consumption and high in efficiency, and its shape and size are easy to be changed, so the conventional permanent magnet motor is widely used in many fields, such as aerospace, national defense, industry, agriculture, manufacturing and many staffs around us. Normally, a permanent-magnet brushless external rotor motor in a ceiling fan can be operated at a low speed of 50 to 300 rpm, and provide high torque. However, low-speed and high torque motor requires large electric current, so its copper loss is largely higher than iron loss. Therefore, it is important to improve the motor efficiency by reducing electric current, and the electric current can be reduced by increasing magnetic flux.
The magnetic flux can be increased by reducing a gap between each permanent magnet. In such a case, the utilization of flux is improved with the increase of the coverage area of the permanent magnets.